Rehabilitation of persons with movement disorders resulting from cerebral palsy, stroke, and other neurological and musculoskeletal impairments poses a significant and challenging health concern. The challenge arises, in part, because movement pathologies can have many sources, including neurological injury, bone deformity, weakness, and muscle contracture. Currently, no tool is available that can quantify the contribution of each of these sources to an individual's movement pathology or predict the functional consequences of common interventions. As a result, the outcomes of treatments are inconsistent, and often unsatisfactory. Forward dynamic simulation offers a powerful methodology for investigating how the elements of the neuromusculoskeletal system interact to produce movement, and could serve as the basis for such a tool. For instance, dynamic simulation could be used to quantify how strengthening a particular muscle or transferring a tendon might affect movement, before any intervention is performed. To date, however, the use of dynamic simulation in clinical applications has been severely limited by the computational expense and high levels of technical expertise needed to generate simulations. The long-term objective of this work is to develop and commercialize innovative software that will make the benefits of forward dynamic simulation realizable by both researchers and clinicians seeking to improve the treatment of movement disorders. Realistic Dynamics, Inc. (RDI) possesses a patent-pending technology that has been shown to dramatically reduce the computational cost of generating muscle actuated simulations of slow, constrained movements such as bicycle pedaling. Aim I will confirm the feasibility of generating muscle-actuated simulations of more complex movements that involve large joint angular velocities, intermittent contact with the environment, and balance. Aim 2 will demonstrate the capability of the software to generate biomechanically valid simulations of pathological gait using less than one hour of computer time. The resulting tool will greatly enhance traditional movement analysis by providing a rigorous theoretical framework for diagnosing the causes of movement disorders and predicting the consequences of treatments.